Progression through developmental stages requires complex interactions of transcription factors and regulatory elements to achieve correct temporal and spatial patterns of requisite gene expression. Biochemical and genetic studies have implicated epigenetic modifications of chromatin structure as an important mechanism in regulation of gene transcription. Alteration of nucleosome conformation and/or position (termed chromatin remodeling) within gene regulatory elements serves to promote or restrict gene expression through regulating accessibility of trans-acting transcription factors. Mechanistically, modification of local chromatin structure is achieved, in part, through the activity of multi-subunit protein complexes that utilize the energy of ATP hydrolysis to disrupt nucleosome conformation and position. One important class of mammalian ATP- dependent nucleosome remodelers is that of the SWI/SNF-related family, which consists of large, multi-protein complexes that utilize either brahma (BRM) or brahma-related gene 1 (BRG1) as the catalytic subunit. Biochemical studies on the human SWI-SNF-related complexes and its yeast counterpart have demonstrated the ability of these complexes to disrupt histone-DNA contacts and reposition nucleosomes in an ATP-dependent manner. Consequently, the SWI/SNF family of complexes functions to render nucleosomal DNA more accessible to transcription factors and restriction enzymes. Mammalian SWI/SNF complexes can be grouped into two major subfamilies, BAF (Brahma-related-gene 1 (BRG1)-associated factor) and PBAF (polybromo- associated BAF). Although the BAF/PBAF complexes share many common subunits; they are distinguishable by the presence of four unique subunits. Although gene-targeting studies in the mouse have implicated SWI/SNF-related complexes in developmental processes, no studies have been done to distinguish the in vivo functional differences between the subfamilies of complexes. To test the hypothesis that the unique subunits are essential for mediating distinct BAF/PBAF cofactor activities at select sites on chromatin in a gene- or cell type-specific manner, a series of novel genetic experiments in mouse are proposed to distinguish complex function by examining phenotype at different times and in various tissue types during development. Public Health Relevance: Biochemical studies on chromatin remodeling complexes have demonstrated their ability to disrupt histone- DNA contacts and reposition nucleosomes. Consequently, these complexes are critical in regulating global gene expression. Genetic experiments are proposed to elucidate the biological specificity of these complexes and the abnormal outcomes that lead to disease states when inappropriately expressed.